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ADM663A/ADM666A

GENERAL INFORMATION

The ADM663A/ADM666A contains a micropower bandgap

reference voltage source; an error amplifier, A1; three compara-

tors, C1, C2, C3, and a series pass output transistor. A P-chan-

nel FET and an NPN transistor are used on the ADM663A

while the ADM666A uses an NPN output transistor.

CIRCUIT DESCRIPTION

The internal bandgap reference is trimmed to 1.3 V

± 30 mV.

This is used as a reference input to the error amplifier A1. The

feedback signal from the regulator output is supplied to the

other input by an on-chip voltage divider or by two external re-

sistors. When VSET is at ground, the internal divider tap between

R1 and R2, provides the error amplifier’s feedback signal giving

a +5 V output. When VSET is at VIN, the internal divider tap be-

tween R2 and R3 provides the error amplifier’s feedback signal

giving a +3.3 V output. When VSET is at more than 50 mV

above ground and less than 50 mV below VIN, the error ampli-

fier’s input is switched directly to the VSET pin, and external

resistors are used to set the output voltage. The external resis-

tors are selected so that the desired output voltage gives 1.3 V

at VSET.

Comparator C1 monitors the output current via the SENSE in-

put. This input, referenced to VOUT(2), monitors the voltage

drop across a load sense resistor. If the voltage drop exceeds

0.5 V, then the error amplifier A1 is disabled and the output

current is limited.

The ADM663A has an additional amplifier, A2, which provides

a temperature proportional output, VTC. If this is summed into

the inverting input of the error amplifier, a negative temperature

coefficient results at the output. This is useful when powering

liquid crystal displays over wide temperature ranges.

The ADM666A has an additional comparator, C4, that com-

pares the voltage on the low battery input, LBI, pin to the inter-

nal +1.3 V reference. The output from the comparator drives an

open drain FET connected to the low battery output pin, LBO.

The low battery threshold may be set using a suitable voltage

divider connected to LBI. When the voltage on LBI falls below

1.3 V, the open drain output LBO is pulled low.

50mV

ADM663A

SHDN

GND

SET

OUT1

OUT2

SENSE

1.3V

0.5V

IN–50mV

0.9V

Figure 1. ADM663A Functional Block Diagram

Both the ADM663A and the ADM666A contain a shutdown

(SHDN) input that can be used to disable the error amplifier

and hence the voltage output. The power consumption in shut-

down reduces to less than 9

µA.

50mV

ADM666A

SHDN

GND

LBO

SET

OUT

SENSE

1.3V

0.5V

IN–50mV

1.3 V

LBI

Figure 2. ADM666A Functional Block Diagram

Circuit Configurations

For a fixed +5 V output the VSET input is grounded and no ex-

ternal resistors are necessary. This basic configuration is shown

in Figure 3. For a fixed +3.3 V output, the VSET input is con-

nected to VIN as shown in Figure 4. Current limiting is not be-

ing utilized so the SENSE input is connected to VOUT(2).

GND

ADM663A

ADM666A

SENSE

OUT(2)

SET

SHDN

+5V

OUTPUT

+6V TO +16V

INPUT

Figure 3. A Fixed +5 V Output

GND

ADM663A

ADM666A

SENSE

OUT(2)

SET

SHDN

+3.3V

OUTPUT

+4.5V TO +16V

INPUT

Figure 4. A Fixed +3.3 V Output

Output Voltage Setting

If VSET is not connected to GND or to VIN, the output voltage is

set according to the following equation:

OUT = V SET ×

( R1

+ R2)

where VSET = 1.30 V.

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